A significant amount of research and development is being done to achieve ultra-deep desulfurization of transportation fuels, such as gasoline and distillates such as diesel and jet fuel. The sulfur content in the transportation fuels is considered an environmental concern primarily because, upon combustion, sulfur can be converted to SOx, which not only contributes to acid rain, but also poisons the catalytic converter for exhaust emission treatment. Because SOx can, under certain circumstances, have harmful effects on people and the environment, the U.S. Environmental Protection Agency (EPA) classifies it as a criteria pollutant. In order to reduce the sulfur pollutant at the source, EPA regulations required reduction of the sulfur content in motor gasolines from an average of 300 wppm to 30 wppm by 2006 and in diesel fuels from 500 wppm to 15 wppm by 2006. Further reduction of sulfur from 30 wppm in motor gasolines and from 15 wppm in diesel fuels to 1 wppm or below is postulated to eventually be required for developing advanced gasoline-/diesel-based fuel cell transportation, as well as ultra-clean liquid fuel-based stationary and portable fuel cell systems.
Currently, sulfur removal from various liquid hydrocarbon streams is achieved by catalytic hydrodesulfurization (HDS) processes at temperatures ranging from about 300° C. to about 400° C. and about 3 MPa to about 6 MPa hydrogen pressure with relatively high hydrogen consumption. One criticism of conventional (catalytic) HDS technology is that, in order to reduce the sulfur content of a transportation fuel from 15 wppm to less than 1 wppm, the catalyst bed volume, or the catalyst activity, must be increased by at least about 65% compared to those values from current refinery catalysts, because the remaining sulfur compounds in the fuels, such as commercial ultra-low sulfur diesel, are the most refractory sulfur compounds (i.e., the most difficult to remove). It is well known that increases in both the reactor volume and the catalyst volume are very costly. Working at relatively high temperatures and relatively high pressures can also limit conventional HDS processes in on-site and on-board desulfurization applications, due to the complication and safety of the overall process. Furthermore, costs of supplying, purifying, and recovering hydrogen gas for such HDS processes are not negligible. Therefore, it is desired to develop other technologies for the ultra-deep desulfurization of transportation fuels.
Various relatively new approaches for ultra-deep desulfurization of liquid hydrocarbon fuels have been reported. Among them is a technology wherein sulfur species are adsorbed on nickel-based sorbents, because of their relatively high capacity and selectivity, without requiring the use of hydrogen gas. For example, U.S. Patent Application Publication No. 2008/0099375 teaches a material comprised of a mixture of nickel phosphides Ni2P, Ni12P5, and Ni3P deposited on silica support at relatively high dispersion that can be used for ultra-deep desulfurization of diesel fuel. The sulfur content is taught as being reduced to the levels of residual sulfur of about 1 ppm and less. The ability of nickel phosphides for conversion of hydrocarbons existing in diesel fuel to carbonaceous deposits is believed to be low. Therefore, the material can be regenerated in a reducing atmosphere. But the Ni compositions and process of the '375 publication have two disadvantages. First, the single-pass sulfur capacity does not exceed 1.6 grams per 100 grams of sorbent. And second, the relatively low adsorption rate of sulfur compounds from a hydrocarbon stream by nickel phosphide phases is not best compatible with increased LHSVs (e.g., at least 5 hr−1) during continuous desulfurization.
Therefore, there exists a need in the art for improved materials and processes for removing substantially all sulfur moieties from transportation fuels.